In the processing of petroleum hydrocarbons and feedstocks such as petroleum processing intermediates, and petrochemicals and petrochemical intermediates, e.g., gas, oils and reformer stocks, chlorinated hydrocarbons and olefin plant fluids such as deethanizer bottoms, the hydrocarbons are commonly heated to temperatures of 100.degree. to 2000.degree. F., frequently from 600.degree.-1000.degree. F. Similarly, such petroleum hydrocarbons are frequently employed as heating mediums on the "hot side" of heating and heating exchange systems.
During such heat processing, and even during ambient temperature transportation and storage, sediment, sludge and/or gummy masses often form with undesirable results. The so-formed sediment, sludge or gums may cause clogging of equipment or fouling of processing equipment (such as heat exchangers, compressors, furnaces, reactors and distillation systems). Oftentimes, the gummy masses or sediment are catalytically formed by the undesirable presence of metallic impurities such as copper and/or iron that are present in the petroleum hydrocarbon or petrochemical.
In the hydrocarbon processing industry, there are several environments where the need for protection against sediment and gum formation is felt. For example, in a refinery, the crude unit has been the focus of attention, primarily because fuel usage directly impacts on processing costs. Chemical additives have been successfully applied at the heat exchangers, both downstream and upstream from the desalter, on the product side of the preheat train, on both sides of the desalter makeup water exchanger, and at the sour water stripper.
The need to inhibit or minimize gum and sediment formation is also felt in conjunction with unsaturated and saturated gas plants such as refinery vapor recovery units, in catalytic cracker units both at the vacuum unit and at the cracker itself, and in heavy oil treating and cracking units.
One especially troublesome area prone to gum and sediment formation is that of the hydrodesulfurizer (H.D.S.) process. Hydrodesulfurization is designed to improve the qualities of a wide range of petroleum stocks by removing sulfur, nitrogen and heavy metallic contaminants and also to saturate the petroleum stocks with hydrogen. Feedstocks to such units may comprise naphthas, kerosene, fuel oils, diesel fuels and residual fuels.
Common hydrodesulfurization applications include pretreatment of catalytic reforming feedstocks and desulfurization of fuel oils. Reformer feedstocks are processed in a hydrodesulfurizer to remove sulfur, nitrogen and arsenic which are poisonous to the reforming catalyst. Fuel oils are upgraded in a hydrodesulfurizer by removing mercaptans and sulfur which cause foul odors and pollution.
The main steps in a HDS process are: feedstock preheating, catalytic reaction, and product purification. In the preheating stage of the process, feed/effluent exchangers normally heat feedstock from ambient to about 450.degree.-500.degree. F. Hydrogen may be added to the feedstock either prior to the exchangers or after. The degree of vaporization varies depending on temperature, feedstock, pressure, and hydrogen content. During the preheating stage, the reactor heats the feed from the preheat effluent temperature to the reactor inlet temperature of about 650.degree. F.
In the reactor section of the HDS unit, a catalyst, such as a Ni-Mo, Co-Mo, or Ni catalyst is normally held in a fixed bed. Metals are retained by the catalyst without seriously affecting its activity over long periods. Sulfur, nitrogen and oxygen compounds are decomposed to the corresponding hydrocarbon with liberation of H.sub.2 S, NH.sub.3 and water. If organic chlorides are present, HCl is formed.
The following equations illustrate the reactions in the reactor section of an HDS unit EQU RSH+H.sub.2 .revreaction.RH+H.sub.2 S (1) EQU RCl+H.sub.2 .revreaction.RH+HCl (2) EQU 2RN+4H.sub.2 .revreaction.2NH.sub.3 +RH (3) EQU ROOH+2H.sub.2 .revreaction.RH+H.sub.2 O (4)
Typical operating conditions for the hydrodesulfurization reactions are:
______________________________________ Temperature, .degree. F. 600-780 Pressure, psig 600-3000 H.sub.2 Recycle rate, 1500-3000 SCF/barrel Fresh H.sub.2 makeup, 700-1000 SCF/barrel ______________________________________
In the HDS purification section, cooling water is used to quench the reactor effluent prior to product separation. The separator or flash drum allows the hydrogen, H.sub.2 S, and NH.sub.3 to flash overhead allowing the liquid process hydrocarbon to continue as bottoms. Water can be removed from the separator drum(s) by level control. The stripper or fractionator, as it is sometimes referred to, uses heat to strip off remaining sour gases. The heat source can be in the form of a stripping steam, a thermal syphon reboiler, or a fired reboiler. The stripper bottom leaves the unit as a final effluent, while the overhead vapors go to an amine contactor and the overheat liquids may go to sour water stripping.
HDS units have become an increasingly important part of refinery processes over the last few years. Removal of sulfur and metals from the feedstock affords important protection for the expensive catalysts used in reformers, cat crackers, and hydrocrackers. Also, air quality regulations seeking to lower the allowable sulfur content in airborne emissions coupled with the use of high sulfur content crudes emphasizes the need for such HDS units.
In addition to use to inhibit sediment and gum formation in HDS units, the present invention can be used in pyrogas units wherein higher molecular weight hydrocarbons, such as those in gas oils, are either catalytically cracked or thermally cracked.
Petrochemical systems, like the petroleum refinery systems noted above, also are adversely affected by gum and sediment accumulation in the process fluid. For example, such problems have been encountered in ethylene and styrene plants. In ethylene plants, furnace gas compressors, fractionating columns and reboilers have all experienced these problems. In butadiene plants, absorption oil fouling and distillation column and reboiler fouling provide troublesome problems that must be overcome to provide process efficiencies.
Accordingly, there is a need in the art to provide for a chemical additive treatment that is adapted to inhibit gum and sediment formation in a liquid hydrocarbonaceous medium. There is also a need for such a treatment that is capable of performing its intended function during the high temperature 100.degree.-2000.degree. F. heat processing of such mediums in accordance with refinery and petrochemical processes. An even more specific need exists for a treatment that is effective in heretofore troublesome processes such as HDS processes, pyrolytic gasoline processes and in butadiene plants.